encasing UG duct w/ rebar or metal ring

[DENVER STEVE]

I am looking for info about encircling a pvc duct w/ rebar (or metal) when you have all three phases within the duct plus a ground cable.
two cases we were held up on;
1-holding a pvc riser in place w/ a metal conduit clamp to unistrut;
2-at the end of the first section of a duct bank pour on a slope, used metal ez form - cut holes in it and slipped it over the end of the ducts to keep concrete from flowing on down the duct bank.

cable in the ducts - 15kV @ ~350A

trying to find some hard literature to give the client's inspector.

 

[don_resqcapt19]

You should not encirlce the conductors of an AC circuit with ferrous metal unless you encircle all of them. If the duct bank raceways have all of the phases in each raceway you won't have any problems. If you are installing a single phase in each raceway, you can expect heating to occur where the raceways are encircled with ferrous metal. With 350 amps of current, this heating could be enough to cause cable failure.